What is public good?
A good that anyone can use without paying and that one person's use does not reduce for the next person.
Public good diagram example
A public good is something that nobody can be excluded from using and that is not used up when one more person uses it. National defense, clean air and street lighting fit this description. Public goods matter because private sellers cannot easily charge for them, so without shared funding they tend to be underprovided.
How it works
Two properties define the idea. Non-excludability means you cannot keep non-payers out, and non-rivalry means your use leaves just as much for everyone else. Because anyone can benefit without paying, each person has a reason to wait for others to cover the cost. This is the free-rider problem, and it is the feedback loop at the center of the topic: the more people expect others to pay, the less gets paid in total.
Communities solve this by making payment compulsory, for example through taxes, or by finding a way to bundle the good with something that can be charged for. The amount of the good that results depends on how much is collected, which is a decision you can change and observe.
Example
Say a town of 1,000 households would each gain 5 per month from a flood barrier that costs 3,000 per month to maintain. The total benefit is 5,000, which is more than the cost, so the barrier is worth having. If payment is voluntary and each household hopes the others will pay, the town may collect only 1,000 and the barrier falls into disrepair. If the town collects 3 from every household, it raises exactly 3,000 and every household still comes out 2 ahead. These figures are an illustration.
How to model it in Machinations
Create a Source called Contributions that feeds a Pool called Barrier fund, and a Drain for monthly upkeep. Add a Register for the barrier's condition that rises when the fund covers upkeep and falls when it does not, with a state connection from the fund to that Register. Make the contribution rate a Lever, one version voluntary and one mandatory. Play both settings and chart the fund and the condition to see where voluntary payment stops covering the cost.
See public good in a system you can Play
Build the rules in a Machinations diagram, change a value and Play it to watch how the system responds.